(CNN) -- A former Florida police officer convicted of multiple murders in the 1980s is scheduled to be executed Tuesday, and his attorneys are still working to save his life.

Manuel Pardo, 56, was convicted of nine counts of first-degree murder in 1988 and was sentenced to death.

Pardo's attorneys argued in federal court Monday that Florida's recent change in the drug combination it uses for lethal injections would violate their client's civil rights. Attorney William McKinley Hennis III told U.S. Judge Timothy Corrigan that if the drugs were to be improperly mixed, the anesthetic effects would be compromised.

"Manny Pardo would be the first inmate to be executed using that new lethal injection protocol," Hennis said.

Corrigan denied the complaint, and Hennis said he would appeal the ruling.

In another legal filing, the Supreme Court of Florida denied Pardo's argument that he should have never been tried in 1988 because he was incompetent to stand trial. David Waksman, who prosecuted the case, dismissed the claim, saying, "He was just a cold-blooded killer who used to be a cop."

Pardo's life started on the other side of the law, according to court documents filed by his attorneys. At age 17, he enlisted in the Navy and served honorably served from 1974 to 1978. In 1981, Pardo re-enlisted in the Marines and remained a reservist while he worked for the Florida Highway Patrol and the Sweetwater, Florida, Police Department.

In 1985 Pardo was fired from the police department and left the reserves when, according to court documents, "he falsely testified in court about police corruption in the Bahamas." Hennis said the stress of losing his job, compounded with a serious undiagnosed disease, turned him into "someone he was not," a killer.

During his trial, Pardo took the stand and admitted to the murders. "He came up with this vigilante story," recalls Waksman. "He said, 'I'm ridding the community of this vermin and technically it is not murder because they're not human beings.'"

The only chance Pardo has to win on the competency issue is if the United States Supreme Court decides to hear his appeal.

The execution is scheduled for 6 p.m. Tuesday at Florida State Prison in Starke, Florida.

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